This invention relates to the isolation of high quality N-benzyloxycarbonyl-L-aspartic acid (Z-Asp) which is suitable for use in the production of L-aspartyl-L-phenylalanine methyl ester(aspartame), an artificial sweetener. In particular this invention describes a method of preparing large crystals of relatively uniform size and high quality by acidifying an alkaline aqueous solution of the dialkali metal salt of Z-Asp at high temperatures.
Aspartame is known to be about 160 times sweeter than sucrose in aqueous solution. Thus, the use of aspartame as a low-calorie sweetener makes it a highly desirable end product. Aspartame is generally prepared from N-benzyloxycarbonyl-L-aspartic acid. In view of the end use of the aspartame in food products as a sugar substitute, the Z-Asp must be as pure as possible and substantially free of by-products. These impurities, such as the dipeptide, N-benzyloxycarbonyl aspartyl aspartic acid (Z-Asp-Asp), benzyl alcohol, benzyl cloride, benzaldehyde, dibenzyl carbonate and sodium chloride are generally formed during or prior to the formation of the Z-Asp.
The reaction of benzyl chloroformate (BCF) with L-aspartic acid (L-AA) to yield Z-Asp has been well known for a number of years. The chemical literature discloses that Z-Asp may be synthesized by the condensation of L-aspartic acid with benzyl chloroformate in an alkaline medium. Prior to 1981, the processes described in the literature did not mention the reaction conditions needed to produce Z-Asp with relatively small amounts of by-products.
U.S. Pat. No. 4,293,706 which issued on Oct. 6, 1981 to Gorman, et al. taught that Z-Asp can be prepared substantially free of Z-Asp-Asp by reacting benzyl chloroformate with the disodium salt of L-aspartic acid in an alkaline aqueous system within a specific pH range of between 10.75 and 11.75, and preferably 11.50 to 11.75 at 20.degree.-25.degree. C. After the reaction is completed, the reaction mixture is acidified with cooling. While maintaining the reaction mixture at 5.degree.-10.degree. C., concentrated acid is added until a pH of 1.5 to 2.5 is reached. This converts the Z-Asp dialkali metal salt to the free acid.
U.S. Pat. No. 4,345,091 issued on Aug. 17, 1982 to Sugiyama, et al. claims that high yields of Z-Asp can be prepared by allowing benzyl chloroformate to react with the sodium or potassium salt of L-aspartic acid and by carrying out the reaction with the pH maintained within the specific range of 12.0 to 13.5 throughout the reaction. Sugiyama, et al. teach maintaining the temperature of the reaction mixture at 10.degree.-30.degree. C. for 3 hours and they also suggest the use of organic solvents to remove any impurities in the system. After separating the organic layer from the reaction mixture, the aqueous layer is cooled and acidified with hydrochloric acid whereby Z-Asp is crystallized. In the sole working example the pH is adjusted to 1 with HCl, and the reaction mixture is allowed to stand overnight at 5.degree. C.
Reaction conditions are very important in the production of a high purity Z-Asp in good yield. However, methods of separation and crystallization are equally important to obtain good products. When a Z-Asp aqueous reaction mixture is acidified with a mineral acid such as hydrochloric acid at low temperatures, the Z-Asp separates at about pH 3.5 as a thick heavy oil which becomes more viscous upon further acidification and it finally solidifies to a solid mass upon standing or upon agitation at the lower pH.
In a usual crystallization system which is initially homogenous, crystals are formed and separated directly from the solution and the impurities remain in the mother liquor. In the cases of patented Z-Asp processes the Z-Asp separates from the solution as an oily layer. According to the rule of thumb that like dissolves like, this oily layer carries with it most of the organic impurities present in the solution. In other words, impurities are dissolved and concentrated in the Z-Asp oily layer from which the Z-Asp crystallizes. The result is that simple crystallization cannot produce a high quality product directly from the reaction mixture. The quality of the product can be improved by extracting the reaction mixture with an organic solvent or by recrystallization of the crude product. These methods are time-consuming and cause significant loss in yield.
Both the Gorman, et al. and Sugiyama, et al. patentees provide suitable reaction conditions within narrow specific pH ranges for preparing a relatively pure Z-Asp with relatively small amounts of Z-Asp-Asp by-product. The patents do not discuss crystal size of the Z-Asp; nor do they suggest ways to improve yield, purity and product properties by increasing the size of the Z-Asp crystals or by any improvement in the manner of crystallization. Accordingly, it is desirable to provide a method for preparing Z-Asp crystals of increased crystal size and increased purity and high yield. The larger crystals are desirable because they are less dusty, they are more easly handled than smaller crystals, and they do not adsorb moisture as readily as small crystals on storage.